This invention relates to a method of density correction as applied to image recording in a dry system such as image recording of a type that employs recording materials which are not subjected to wet processing, especially thermal recording materials which produce densities by color formation with dyes as in thermal recording with a thermal head, and light-sensitive and/or thermal recording materials or thermally processable light-sensitive recording materials in the image recording with at least one laser beam and thermal development or in the thermal recording with heat of the laser beam or the thermal head. The invention also relates to an image recording apparatus.
Well known in the art are a digital radiographic system using a stimulable phosphor sheet, as well as CT (computer tomography), MR (magnetic resonance imaging) and other medical image recording apparatus. Conventionally, these apparatus have been operated by a "wet system", in which silver salt photographic materials carrying taken pictures or recorded images are subjected to wet processing to yield reproduced images. As an alternative method, a "dry system" has recently drawn increasing attention and an apparatus using thermal recording materials or thermally processable light-sensitive recording materials is one of such recent approaches. A problem with this dry system is that in order to meet the requirement of the medical industry for producing high-quality images, these materials must have high sensitivity.
Conventionally, in addition to organic silver salt, dyes are used as color forming means to produce densities in various image recording processes of the dry system. For example, thermal recording films (hereunder referred to as "thermal materials") comprising a thermal recording layer on a film substrate are commonly used to record images produced in diagnosis by ultrasonic scanning. This recording method, commonly referred to as thermal image recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, the use of the thermal image recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, the thermal recording apparatus uses the thermal head having a glaze in which heat generating resistors corresponding to the number of pixels of one line are arranged in one direction and, with the glaze a little pressed against the thermal recording layer of the thermal film, the thermal film is relatively moved in a direction approximately perpendicular to the direction in which the heat generating resistors are arranged, and the respective heat generating resistors of the glaze are heated in accordance with the image data to be recorded to heat the thermal recording layer imagewise, thereby accomplishing image reproduction.
Not only in this thermal recording system but also in various other image recording apparatus such as laser printers and printing platemaking apparatus, image data (image information) on the image to be recorded are received from an image data supply source such as a diagnostic measuring apparatus or an image reading apparatus and various image processing steps such as sharpness enhancement and shading correction are performed on the received image data to produce image data for final image recording and the desired image is accordingly recorded on a recording material.
One of the important requirements of the image recording apparatus is that images of predetermined densities be always produced in accordance with the image data received from the above-mentioned image data supply source. Take, for example, an apparatus that receives image data as 10-bit digital data and which performs image recording on the basis of the received image data; if digital data representing 300 corresponds to a density (D) of 1.2, it is required that the apparatus produce an image at the density of 1.2 whenever it receives digital data representing 300.
However, image recording apparatus have individual differences and the density of the recorded image is also influenced by such factors as the environment in which the apparatus is installed; therefore, it is impossible for all apparatus to output images of predetermined densities in accordance with the supplied image data.
Under the circumstances, conventional image recording apparatus are adapted to be such that the density correcting conditions for outputting images of predetermined densities according to image data are set for each apparatus and that an output image is produced after the image data are corrected or density corrected (the step generally referred to as "calibration") in accordance with the thus set density correcting conditions.
As a further problem, the condition of a particular image recording apparatus is variable not only with the progress of the recording operation but also with aging and, hence, it is difficult to ensure that images of predetermined densities are kept produced over an extended period. Consider, for example, the aforementioned thermal recording apparatus; as repeated recording is performed, the glaze on the thermal head is stained or worn or operating parameters such as the resistances of heating elements will change and, in addition, the environmental conditions (in particular, temperature) will also change; as a result, given the same image data, the density of output image will vary with aging. It is therefore necessary that the density correcting conditions be updated periodically.
Conventionally, in order to set (or update) the density correcting conditions, an individual image recording apparatus is adapted to output a chart for setting the density correcting conditions (i.e., a density correcting chart) which has images of various densities recorded thereon, And the densities of images recorded on the chart are measured with a densitometer and compared with the densities of the images which are to be recorded with the apparatus (namely, the image density in accordance with the image data); the results of the comparison are plotted on a graph to construct a calibration curve.
However, densitometers in common use today are designed to operate on the basis of the images produced by using silver halides as means of color formation in the wet system and correct data can not be obtained consistently if such densitometers are used to measure the densities of images produced by color formation with dyes as in thermal recording systems.
Stated more specifically, the sensitivities of densitometers in common use today are generally variable with wavelength; however, in color formation with silver halides in the wet system, absorption of light is fixed and independent of wavelength, so the results of image density measurement are consistently in agreement with the visual density. On the other hand, color formation with dyes involves the wavelength dependency of absorption and the results of image density measurement with a densitometer will differ from the visual density.
Consider, for example, two images, one being an image having a visual density of 2.0 that has been produced as a result of color formation with silver halides in the wet system and the other being an image of the same visual density which has been produced by color formation with dyes. If these images are measured with the same densitometer, the image density produced by color formation with silver halides in the wet system takes the value 2.0 which is equal to the visual density; however the image density produced by color formation with dyes is generally lower (e.g. 1.8). Such differences in the data of measurement also occur between densitometers.
Under the circumstances, the image recording system that uses a recording material capable of color formation by means of dyes has had the problem that if the density correcting conditions are set in accordance with the results of densitometer measurements on the density correcting chart, the correct density correcting conditions cannot be set and it is impossible to record appropriate and high-quality images which comply with the supplied image data, as typically exemplified by the increase in the overall density of the recorded image.
Various known methods are available for setting the density correcting conditions, such as by using a dedicated algorithm (i.e., a density correcting algorithm) and the methods in ordinary use are so set that they are adapted to a system in which the image density increases gradually from a recording energy value of zero as simulating the image recording that uses silver salt photographic materials in the wet system and in which the image density increases gradually with the increasing amount of recording illumination from zero.
However, in a thermal recording system of the type described above, the recording energy or thermal energy has a threshold in color formation and no color forms if the supplied recording energy is below the threshold but once it is reached, color formation takes place abruptly and the slope of the increase in image density is also steep.
Therefore, if the density correcting conditions are set by the ordinary methods, satisfactory gradation cannot be reproduced and particularly in the highlight density area (the brightest area) near the threshold, the gamma value increases and delicate tones cannot be reproduced, thereby making it impossible to record images of high quality.
In thermal recording apparatus, pulse width or pulse numbers modulation is performed with respect to the image data and the pulses thus modulated are employed to control the generation of heat from the individual heating resistors such that a predetermine gradation is represented in the recorded image for a predetermined density range.
A typical method of controlling the gradation of the image to be recorded will now be described with particular reference to pulse width modulation.
FIG. 13 shows the concept of a circuit for performing the control of heat generation from individual heating resistors by means of pulse width modulation. Shown by 118 in FIG. 13 is control circuit that is provided for each of the heating resistors 120 and it is composed of a pulse generator 122 which, in response to a control clock, generates pulses having a pulse width associated with the image data of interest and a switching element 124 which, in response to the pulses delivered from the pulse generator 122, controls the application of an electric current to a particular heating resistor 120.
The operation of the control circuit 118 may be understood from the timing chart shown in FIG. 14; the pulse generator 122 counts heating control clocks and outputs pulses having widths associated with the image data. In the illustrated case, the switching element 124 is turned on for a time period corresponding to the width of "high-level" pulses being produced from the pulse generator 122 and, conversely, it is turned off for a time period corresponding to the width of "low-level" pulses. Thusly, the heating resistor 120 is supplied with an electric current for a predetermined time by means of the switching element 124 and heated to a predetermined period temperature in accordance with this time period of energization.
Consider, for example, a thermal recording apparatus which, as shown in FIG. 15(a), reproduces 1,024 tones for an image density range of D=0-3. If the pulse width required for recording an image density of D=3 is expressed as a maximum heating time, the pulse generator 122 will output pulses ranging in width from zero to the maximum heating time depending upon specific image data. In this case, the density of the image to be recorded ranges from D=0 to D=3 and the gradation of the image will accordingly change from zero to 1,023 tones.
Therefore, the oscillation frequency of heating control clocks is determined such that the pulse generator 122 will count 1,023 heating control clocks when the width of pulses being produced from the pulse generator 122 has become equal to the maximum heating time.
Thus, in a thermal recording apparatus of the type that uses a thermal head capable of being controlled by the control circuit 118 which adopts the pulse width modulation system, the pulse width is changed in accordance with the image data of interest, thereby controlling as required the gradation for the recorded image.
In fact, however, due to the instrumental variability among thermal recording apparatus such as the scattering of the resistance values of the respective heating resistors 120 or on account of the differences in operating parameters such as the sensitivity of individual thermal films, the maximum energy required to record an image of the maximum density will change and so does the maximum heating time required. If, in spite of these changes, one intends to reproduce a maximum image density of D=3, as many as 1,024 tones have to be assigned to the required maxim heating time and if the required heating time is short, the number of tones will decrease as explained below.
If the required maximum heating time becomes short as shown in FIG. 15(b) for the reason set forth in the preceding paragraph, the number of heating control clocks being counted by the pulse generator 122 must be curtailed in order to reduce the width of pulses being produced from the pulse generator 122; but then, the number of tones that can be represented in the recorded image may decrease to, for example, 800 tones in the illustrated case, thereby making it impossible to represent the required 1,024 tones.
Also available in the art is a thermal recording apparatus that performs pulse intensity (amplitude) modulation with respect to the supply power to the head such that the intensity modulated image data pulses are used to control the generation of heat from the individual heating resistors 120 such that a predetermined gradation is represented in the recorded image for a predetermined density range.
However, this type of thermal recording apparatus which adopts pulse intensity modulation in controlling the gradation of the recorded image has another problem in that the inevitable need to employ means of adjusting the line voltage adds to the cost of the power circuit.
Various kinds of the problems described above would be present not only in the case of the image recording that employs the recording materials which produce densities by color formation with dyes, but also in the case of the image recording that employs the recording materials in the dry system which is not subjected to the wet processing.